Method of making large surface area filaments for the production of polysilicon in a cvd reactor

ABSTRACT

The bulk polysilicon deposition rate of a Siemens method CVD reactor system having a power supply configured for deposition on a solid rod silicon filament of a specified diameter and length is increased by installing a high surface area silicon filament in the CVD reactor in lieu of the specified solid rod filament, the high surface area filament being dimensionally configured such that it can be used in place of the solid rod filament without reconfiguring or replacing the reactor power supply. The high surface area filament can be tubular, flat, or shaped with radial fins. Existing reactors thereby require only adaptation or replacement of filament supports to be adapted for use of the high surface area filament. The high surface area filament can be grown from silicon melt using the EFG method, so as to maintain a cross-sectional shape within a tolerance of +/−10%.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/413,425, filed Apr. 28, 2006, herein incorporated by reference in itsentirety for all purposes.

FIELD OF INVENTION

This invention relates to the chemical vapor deposition of silicon, andmore particularly to the use of shaped silicon filaments with largerstarting surface areas for deposition than traditional solid slim rods,in CVD reactors of the same general design.

BACKGROUND OF THE INVENTION

Use of polysilicon by the photovoltaic industry has been growing rapidlyand in 2005 this demand was essentially equivalent to the use ofpolysilicon by the microelectronic industry. The anticipated growth rateof the photovoltaic industry is expected to be between 15 to 30% (recentyear growth has been at 30 to 45%) compared to the microelectronicindustry at 7 to 10% which will result in much larger demand ofpolysilicon for the photovoltaic industry. While the silicon wafer costconstitutes approximately 25 to 33% of the PV (photovoltaic) modulecosts, it is less than 5% of the silicon semiconductor device costs inthe microelectronic industry. Therefore, it is imperative to reduce thecost contributions of polysilicon for the photovoltaic industry. The PVindustry has learned to use polysilicon with minor imperfections andslight contamination as one way to contain costs.

One of the most widely practiced conventional methods of polysiliconproduction is by depositing polysilicon in a chemical vapor deposition(CVD) reactor, and is referred to as Siemens method. Referring to priorart FIG. 1, a CVD reactor consists of a base plate 23, chamber wall orquartz bell jar 17. There is incorporated in base plate 23, a gas inlet20 and a gas outlet 21 (can be in the same position), and electricalfeedthroughs 19. A viewing port 22 provides for visual inspection of theinterior or for the temperature measurement.

In the prior art polysilicon manufacturing by CVD, a high-purity siliconslim rod structure or filament is assembled in the form of a hair pin byhaving a cross rod 2 placed horizontally on two long, spaced apart,vertical rods 1 and 3. This structure is mounted and connected so as toprovide a current path between electrical feedthroughs 19. During theCVD process, polysilicon deposit accumulates uniformly on the slim rods;the deposit 41 being shown here partially removed to show the slim rodstructure. Different users employ different methods for joining thehorizontal rod to the vertical rods. One method requires a groove or akey slot at the top of each vertical rod. A small counter bore orconforming fitment is formed at the ends of the horizontal rod so thatit can be press fitted into the grooves to bridge the two vertical rods.

Because of the high purity silicon from which these rods are fabricated,the corresponding electrical resistance of the slim rods is extremelyhigh. Thus it is extremely difficult to heat this silicon “filament”using electrical current, during the startup phase of the process.

Sometimes the slim rods are replaced by metallic rods that are moreconductive and easier to heat with electrical current. This method isreferred to as Rogers Heitz method. However, the introduction of metalinto the chemical vapor deposition process can introduce metalcontamination. This contamination of the polysilicon yield is notacceptable in the semiconductor/microelectronics industry. However, forthe photovoltaic industry the wafers used for fabricating solar cellsare typically doped with Periodic Table group 3 elements, such as boron(B), or group 5 elements, such as phosphorous (P), to make them moreconductive.

Resistivity of pure silicon is a strong function of temperature, rangingfrom 10⁶ ohm·cm for a slim rod at room temperature to 0.01 ohm·cm at1200 deg C. Doped silicon, however shows a different behavior. Dependingon the concentration of the dopant, e.g, Boron, the resistivity willincrease along with the temperature to a certain point, and then becomethe same as an intrinsic silicon slim rod. At room temperature, a borondoped silicon slim rod at 10¹⁸ atom/cm³ is about 0.05 ohm·cm. There issome tolerance for impurities, especially for the dopant ions, whenpolysilicon is used for photovoltaic applications.

A typical prior art reactor for conducting a Siemens-type processincludes a complex array of subsystems. External heaters are used toraise the temperature of the high purity slim rod filaments toapproximately 400° C. (centigrade) in order to reduce their electricalresistivity or impedance to current flow. Sometimes external heating isapplied in form of halogen heating or plasma discharge heating.Normally, a multi-tap electrical power supply is required for theresistance heating of the filaments. It can provide very high voltagesand low current for the early phase heating; and a very high current atrelatively lower voltage for the later phase when the resistivity of therods has been decreased by the higher temperature.

High voltage switching gear is needed for switching between the powerlevel taps. The first process of sending low current at high voltagethrough the filaments continues until the temperature of the filamentsreaches about 800° C. At this temperature, the resistance of the highpurity silicon rods falls very drastically and the high voltage sourceis switched to the low voltage source that is capable of supplying thehigh current. However, since the current drawn by the silicon slim rodsat around 800° C. is of a run away nature, the switching of the highvoltage to low voltage power source needs to be done with extreme careand caution.

During the CVD process, silicon deposits onto the hot surface of thefilaments and the diameter of the resulting silicon rods becomes largerand larger. Under the constant process conditions of gas supply, reactorpressure, the surface temperature of the growing rods (typically 1100degrees C. for using trichlorosilane as the decomposition gas, forexample), the rate of the diameter increase (or the deposition rate interms of micrometer per minute) is more or less constant. The typicalstarting size of the silicon slim rods is about 7 mm with a round orsquare cross section. The size of the metal wire slim rods is evensmaller. Therefore, the production rate in terms of kg per hour is verylow at the initial stage when the silicon rod diameter is small.

In one kind of conventional CVD reactor, high purity silicon isdeposited by reaction of trichlorosilane (SiHCl₃) and hydrogen (H₂) ontosolid slim rods of typically 7 mm diameter. In a typical reactor anarray of slim rods are assembled; this placement is based on radiantheat transfer between the rods, heat losses to the outside wall anddeposition rate of silicon on these slim rods. Faster deposition ratescan result in imperfections in polysilicon productions which is notacceptable to the microelectronics industry; however the photovoltaicindustry has learned to deal with such minor imperfections.

There have been past efforts made to modifying the current CVD reactorswith the intent to simplify the number of slim rods or to increase thedeposition rates, but they have not achieved widespread acceptance asthe new reactors deviated considerably from the conventional reactordesigns and it would be very costly and time consuming to retrofit orreplace existing CVD reactors and optimize all other parameters prior tocommercialization.

SUMMARY OF THE INVENTION

It is an object of the present invention to increase the throughput ofconventional CVD reactors by incorporating silicon shapes, such assilicon tubes, ribbons, or other large surface area filament shapes ofsimilar electrical properties, instead of the conventional solid slimrods, so that the initial surface area for deposition of silicon isincreased. For example, using a tubular silicon filament of 50 mmdiameter rather than the conventional slim rod, the productionthroughput can be increased by 30-40% without compromising the qualityof the product and without significant changes to the reactors. Therequired change to the reactor design to use the alternative filament isso minor that it can be retrofitted to current CVD reactors quickly andat very modest cost. It can even more easily be incorporated into newreactors of the same basic design, with further cost reduction benefits.

Another object of the present invention is to increase the throughput byuse of silicon filament shapes with larger surface areas, with minimumchanges of the existing Siemens reactors designed for silicon slim rodsas the filaments. By choosing the appropriate cross section area of thenew filaments, the high voltage needed for launching the filamentheating will be the same as that used for the slim rod filaments.Therefore, the same power supplies, which are expensive components ofthe reactor system, can be used. This makes the retrofit of existingreactors to use filaments of the invention attractive.

An additional object of the invention is to provide a guideline forchoosing an appropriate starting diameter of a tubular silicon filament,which is dependent on the downtime of the process. The process downtimeis defined as the time between the shut down of power for the depositioncycle (end of the deposition) to the start of passing silicon containinggases in the next run. The downtime includes cooling down of thereactor, purging of the reacting gases, removing of the products,cleaning of the reactor, mounting of the filaments, purging of thereactor, preheating of the filaments (if needed), and heating up thefilaments to the deposition temperature. The typical downtime in theproduction ranges from 6 to 12 hours.

Yet another object of the invention is to provide a cost-effectiveprocess for growing the silicon shapes that are useful as the largesurface area filaments having suitable deposition and electricalproperties. By applying the well-established EFG method and appropriatedies, different sizes and cross section shapes of silicon filament stockcan be grown continuously at a fast rate.

Still another object of the invention is to disclose a high-throughputmethod for growing the silicon shapes by growing multiple lengths offilament stock from the same melt reservoir at the same time usingmultiple dies or a multi-cavity die of the desired cross section shape,size and wall thickness.

An additional object of the invention is to disclose a method forgrowing highly doped shaped filaments such as the tubular filamentsdisclosed. The filaments can be doped either p-type or n-type. As notedpreviously, the use of the doped filaments can eliminate the need of apreheating of the starting filaments by an external heating source, andreduce the voltage needed for launching the heating of the filaments bythe passing the electric current directly through the filaments. Suchdoped filaments enable simplification of the power supply and controlcircuit and reduce the cost of the subsystems for the CVD reactor. Italso reduces the time needed for heating up the filaments.

The invention is especially beneficial when using doped siliconfilaments in the case of building new Siemens-type reactors. The powersupplies can be greatly simplified without the requirement of highvoltage (several thousand volts for the launching stage), and the costof the power supply, which is a major cost component of the reactorequipment, can be significantly reduced.

In summary, the invention of making and adapting relatively largesurface area filaments of similar electrical properties to traditionalslim rods, to reactors designed for and used with slim rods, contributesto cost reductions in both the cost to update existing reactors and thecapital cost of new reactors by enabling the use of substantiallysimilar reactor designs but with important cost savings in specifichigh-cost components; and to higher yields and lower per unit costs.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cut away perspective view of a prior art chemical vapordeposition (CVD) reactor utilizing a slim rod filament as the targetupon which a coating or layer of polysilicon has been deposited byoperation of the Siemens Process within the reactor.

FIG. 2 is a cut away perspective view of a chemical vapor depositionreactor within which a thin wall polysilicon structure of substantiallygreater initially exposed surface area replaces the slim rod of FIG. 1,and upon which a coating or layer of silicon has been deposited byoperation of the Siemens Process within the reactor.

FIGS. 3A, 3B, 3C, and 3D are examples of silicon filaments of variouscross sections suitable for use in the reactor of FIG. 2.

FIG. 4A is a graph of estimated polysilicon production, annualthroughput, as a function of silicon tube filament outer diameters, byassuming different downtimes.

FIG. 4B is a cross section illustration of a tube filament, shown bothat start up having a 2 millimeter wall thickness, and at completionhaving grown by deposition to 120 millimeter diameter.

FIG. 5 is a graph for selecting a suitable wall thickness of the tube inmillimeters and starting tube diameter in millimeters for a tubularfilament of the invention.

FIG. 6 is a simplified cross section view of a method and apparatus forgrowing silicon shapes as filaments for the reactor of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The invention is susceptible of many variations in form and practice.Accordingly, the figures and following description of embodiments of theinvention are to be regarded as illustrative in nature, and not asrestrictive.

According to one embodiment of the invention, shaped silicon filamentsare used to replace the conventional slim rods in a CVD reactor formaking polysilicon. Referring to FIG. 2, one embodiment of the inventionutilizes one or another of silicon tube filament sections 31, 32, 33 asthe starting filament shape in a CVD reactor adapted accordingly,replacing the silicon slim rods 1, 2, and 3 of FIG. 1 in the analogousconventional CVD reactor. Filament sections of other cross sectionshapes, where the shape yields a significantly larger available surfacearea for deposition than a solid core slim rod, are within the scope ofthe invention. Filament sections may be of a mix of cross sectionshapes, such as a flat or ribbon filament bridge section connectingtubular or star shaped vertical sections, so long as their depositionand electrical properties are consistent.

By selecting an appropriate wall thickness and cross section area of thesilicon filament sections, the electrical resistance characteristics canby design approximate those of a slim rod filament. This allows thepower supply that was designed and used for heating the silicon slimrods of the reactor of FIG. 1 to be utilized for heating up the largesurface area filaments of the FIG. 2 reactor.

Using tubular silicon filaments as an example, assuming the same totallength of the filaments in one current loop of a power supply as in aconventional Siemens reactor, the power needed to heat up the filament,thus the maximum voltage required at the launching stage, will bedetermined by the cross-section area and the circumferential or surfacearea of the new filaments based on the energy balance. Those skilled inthe art will appreciate the mathematics required to design analternative large surface area filament profile such as a tubularfilament to have similar electrical properties as a solid rod filament.In the case of tubular filaments, in order to utilize the same powersupply as used for launching and heating the slim rod filaments, thethickness and the outer diameter (OD) of the tubular silicon filamentsmust satisfy the relationship:

4(d _(o)−δ)δ≧d _(o) d _(slim)  (1)

where,

d_(o) is the OD (outer diameter) of the tubular filament,

δ is the thickness of the tube, and

d_(slim) is the diameter of the conventional slim rods.

Therefore, depending on the diameter of the starting tubular siliconfilament, the wall thickness δ, reference W of the silicon tube 31 ofFIG. 4B, should be larger than a specific value as depicted in FIG. 5.For example, if the starting silicon tube OD is 50 mm, the appropriatethickness of such tubular filaments will be about 1.8 mm or more inorder to use the same power supply as would be used for slim rodfilaments of 7 mm diameter.

While a 50 mm diameter tubular filament of about 2 mm wall thickness isdescribed, larger and smaller diameters and larger and smaller wallthicknesses are within the scope of the invention. For example, a usefulrange of wall thicknesses for filaments according to the invention isfrom about 1 mm to 6 mm, or more usefully, from about 1.75 to 3.5 mm, asindicated by FIG. 6. A useful range of tubular filament diameters isfrom about 10 mm and up, practically limited by tube cost and reactorsize.

For using large surface area, shaped silicon filaments with electricalproperties similar to a conventional slim rod filament, in a reactorotherwise suitable for using conventional slim rods, only the electrodestuds, generally consisting of graphite, need to be modified orreplaced, to provide a configuration suitable for holding the newfilaments with the required electrical contact. In one embodiment, thesilicon filament sections, such as illustrated sections 31, 32, 33, areconnected mechanically at their intersecting points to form an invertedU-shaped hair pin, with the horizontal bridge section 32 sitting in agroove or a key slot at the top of the each vertical section 31, 33.Another way to connect the shaped silicon bodies to form a inverseU-shaped is to use components made of silicon or pure graphite tointerconnect the different sections. Other means of making a suitablemechanical and electrical connection between vertical and bridgingsections of a filament of the invention are within the scope of theinvention. This inverted U-shaped filament structure is mounted to theelectrode studs and connected so as to provide a current path between apair of electrical feedthroughs 19. During the CVD process, existingpower supplies are used in the conventional manner to heat thefilaments; polysilicon deposit 41 accumulates uniformly on the exposedsurface of the silicon filament; the deposit layer being shown herepartially removed to expose the silicon filament structure.

The silicon filament shapes can be uniform or non uniform in crosssection and shape over its length, and may include but not be limited tosilicon tubes, silicon ribbons, or elongate silicon bodies with othercross-sections, such as illustrated in the profiles or different crosssections in FIGS. 3A-3D. The main principle is that the starting siliconfilaments have a significantly larger surface area for silicondeposition than a solid core slim rod, with similar electricalproperties. The relatively shorter bridge section may be a solid rod inthe order of a slim rod, so long as the relatively longer verticalsections have the desired larger surface area geometry described, andthe total filament resistance is correct.

Referring again to FIGS. 3A-3D, there are illustrated respectively asilicon tube, a silicon ribbon, a silicon member with a cross orX-shaped cross section, and a silicon member with a star-shaped crosssection. These are illustrative only, and not limiting of the largesurface area cross sections that may be used.

Referring again to FIGS. 4A, 4B and 5, the optimal size of a startingtubular filament, in particular the outer diameter and wall thickness,is determined by several factors, including the availability of the tubesize, the cost of the filaments, the uniformity of the heating of afilaments, the required deposition rate (in terms of mm/hour), theprocess downtime, and of course, the market value of the polysiliconproduct. Generally, the larger the starting filament diameter, thehigher the cost of the filament, and the more difficult it is to heatthe filaments uniformly. The slower the growth rate, or the lower thedowntime, the more benefit a larger diameter starting tube filamentprovides. It is important to point out that there is an optimal outerdiameter for a starting silicon tube filament depending on the growthrate and the downtime, as is illustrated in FIG. 4A.

It is useful to compare the results using a conventional CVD reactorwith 18 U-shaped slim rods and an average deposition rate of 13.6microns per minute with trichlorosilane and hydrogen mixture, to theproduction levels afforded by the invention. The graph of FIG. 4A showsthe silicon production per year (metric tons per year) as a function ofthe outside diameter (OD) of the starting filament in accordance withthe invention, in this case a tube, when such filaments replace theprior art solid slim rods in the conventional CVD reactor. FIG. 4A alsoillustrates the growth time in hours and the effect of the down timerequired between cycles, on the total production yield. Curves a, b, c,d, and e represent the yield curves for operations having downtimes of,respectively, 24, 12, 8, 6, and theoretically 0 hours downtime between,CVD cycles. Referring to FIG. 4B, there is illustrated an exemplarysilicon filament 31 of a tubular geometry, with an initial 2 mm wallthickness and 50 mm OD, which after a full CVD cycle has grown a silicondeposit 41 to the finished final diameter D_(F) of 120 mm.

Data of the type illustrated in FIG. 4A would show that for aconventional 7 mm diameter solid slim rod, the growth time is more than70 hours and the reactor produces less than 231 metric tons per yearwhen the down time between operating cycles was limited to 6 hours. Whenthe solid slim rods are replaced with 50 mm OD tube filaments of theinvention, the growth or CVD times are expected to be about 45 hours.Using the same 6 hour down time curve d for the calculation, 304 metrictons of polysilicon can be produced per year. As can be seen, the use ofa 50 mm OD tubular starting filament in accordance with the inventionyields about 30% more in throughput under the normal downtime of about 6hours.

In a conventional CVD reactor higher growth rates can result in gasentrapment in the deposited silicon which leads to problems in thesubsequent step of crystal growth by the Czochralski process formicroelectronic applications. If the initial surface area of thefilament is larger as taught herein, higher deposition rates can beinitiated and increased without gas entrapment. For photovoltaicapplications after polysilicon production, the subsequent step isusually multi-crystalline ingot growth by a heat exchanger method (HEM)or directional solidification system (DSS) for which minor gasentrapment in silicon is not a problem, enabling yet faster depositionrates. Faster polysilicon deposition rates and greater overallproduction yields will result in significantly lower costs of productionwhich is very important for the photovoltaic industry.

For the microelectronic applications the purity of polysilicon producedhas to be very high, less than 1 part per billion total metallicimpurities. In contrast, a silicon wafer for solar cells has a morerelaxed requirement of less than 1 part per million total metallicimpurities, three orders of magnitude more impurities. In fact, most ofthe silicon for solar cell applications is intentionally doped withboron (B) to about 0.5 parts per million atomic prior to fabricatingsolar cells. Therefore, polysilicon production for photovoltaicapplications is able to utilize filaments of the invention similarlydoped with boron or other suitable dopants; and the resultant lowresistivity aids the direct resistive heating of the filaments duringinitial stages of the polysilicon deposition. This eliminates therequirement of a complex array of subsystems and two power sources; onepower supply that can provide very high voltage and low current, and asecond power supply that can sustain a very high current at relativelylower voltage plus the associated switching circuit. The two powersupplies and related switching circuitry can be replaced with a simplesystem of current supply and temperature controls. This change in designwill result in lower capital equipment costs for new reactors of similardesign, and for retrofit of existing reactors when required. This changeand type of operation avoids cumbersome and time consuming start upprocedures, lowers down time and increases productivity, forapplications where the resulting purity level is acceptable.

The present invention is not restricted to CVD reactors usingpolysilicon deposition involving reaction of trichlorosilane but can beused for reactions involving silane, dicholosilane, or other derivativesor combinations of gases; by replacing solid slim rods with largesurface area geometries and similar electrical resistivity properties inaccordance with the invention.

While tubular filaments are preferred over other variations of largesurface area filaments, the invention is not limited to tubular filamentshapes. Referring to FIG. 5, there is illustrated a method and apparatusfor making the FIG. 3 silicon filament shapes 31, 32, 33 and other largesurface area shapes of suitable cross section in accordance with theinvention. The method is generally described as the EFG (Edge-defined,film fed growth) method. One embodiment of this aspect of the inventionconsists of a silicon melt pool 54, contained in a graphite or quartzcrucible 53. The melt is heated with a resistive or induction heater 55,and replenished with a silicon feeder 56, by feeding silicon solid orliquid 57 continuously. The silicon shape 51 crystallized out of theshaping die 52 is the stock from which filament sections of theinvention are taken. The shaping die material can be graphite or quartz.Other variations of this apparatus and method are within the scope ofthe invention.

In another aspect of the invention, producing the FIGS. 3A-3D and othersilicon filament shapes of the invention consists of using an EFG systemwith multiple shaping dies 52 feed by a common melt pool 54, where thedies may be of the same or different filament cross section geometries.Filament wall thickness tolerances can generally be held to within 10%of the target thickness in the axial direction. Such variation in thetube thickness will be evened out during the CVD deposition processbecause the thin section of the tube will have slightly highertemperature than that of a thicker section, and the higher temperaturewill result in a faster growth of silicon in that area. Thisself-compensation phenomenon is also observed in the CVD process usingconventional slim rod filaments.

One example of the invention is a CVD reactor for bulk production ofpolysilicon consisting of a base plate system that might for example beone plate or a pair of opposing plates, configured with filamentsupports, and an enclosure attachable to the base plate system so as toform a deposition chamber. There is at least one silicon filamentdisposed within the chamber on the filament supports, and an electricalcurrent source connectable to both ends of the filament via electricalfeedthroughs in the base plate system, for heating the filament. Thereis at least one gas inlet in the base plate system connectable to asource of silicon-containing gas, and a gas outlet in the base platesystem whereby gas may be released from the chamber.

The filament has a tubular cross section with an outer diameter of atleast 20 mm and a ratio of wall thickness to diameter of not greaterthan ¼. The starting diameter may be other or greater than 20 mm, forexample it may be in the range of 20-100 mm, and the wall thickness mayrange according. Alternatively, a tubular filament may have a startingouter diameter in the range of 40-80 mm, and a wall thickness in therange of 1.75-6 mm. One tubular embodiment may have a starting diameterof about 50 mm and a starting wall thickness of about 2 mm. The filamentmay be doped with at least one element from one of groups 3 and 5 of thePeriodic Table, whereby its impedance at room temperature is reduced toless than in the order of 10³ ohm·cm.

Another example of the invention is a method for making and using largesurface area filaments in a CVD reactor for the production ofpolysilicon, consisting of heating silicon in a silicon melt pool to amolten state, and growing with the silicon in a molten state by an EFGmethod with a die, a silicon structure consisting of a cross sectionwith an outer circumference of greater than 60 mm and an impedance tothe flow of electrical current ranging from in the order of 10⁶ ohm·cmat room temperature to 0.01 ohm·cm at 1200 deg C.; then disposing atleast one section of the silicon structure between two electrodes withina CVD reactor so that it can function as a filament. Then heating thefilament with electrical current and conducting a CVD process with asilicon-containing gas so that the filament receives a deposit ofsilicon. The die may be a multi-cavity die. There may be doping of thesilicon structure so as to reduce the impedance at room temperature toless than in the order of 10³ ohm·cm.

Yet another example of the invention is a method for producingpolysilicon consisting of using a silicon-containing gas and a CVDreactor, disposing in the CVD reactor a tubular silicon filament havingan outer diameter in the range of 40 to 60 mm and a wall thickness inthe range of 1.75 to 6 mm, and conducting a CVD process with thesilicon-containing gas wherein the tubular silicon filament is heated byelectrical current so that it receives a growing deposit of siliconuntil the CVD process is terminated.

Other and various examples and embodiments of the invention will bereadily apparent to those skilled in the art from the description,figures, and claims that follow.

What is claimed is:
 1. A CVD reactor for bulk production of polysiliconcomprising: a base plate system configured with filament supports; anenclosure attachable to said base plate system so as to form adeposition chamber; a power supply adjusted and configured for use witha solid rod silicon filament having a specified solid rod diameter, aspecified solid rod length, and a specified solid rod surface area; atleast one high surface area silicon filament disposed within saidchamber on said filament supports, said high surface area siliconfilament having a length substantially equal to the solid rod length anda surface area greater than the solid rod surface area, while also beingusable in the CVD reactor without reconfiguring or replacing the powersupply of the CVD reactor; electrical feedthroughs in said base platesystem, said electrical feedthroughs being adapted for connection of thepower supply to both ends of said high surface area silicon filament; agas inlet in said base plate system connectable to a source ofsilicon-containing gas; and a gas outlet in said base plate systemwhereby gas may be released from said chamber.
 2. The CVD reactor ofclaim 1, wherein the cross sectional shape of the high surface areasilicon filament includes outwardly projecting fins.
 3. The CVD reactorof claim 1, wherein the high surface area silicon filament is tubular.4. The CVD reactor of claim 1, wherein the high surface area siliconfilament comprises two vertical filament segments electrically connectedby a bridge segment.
 5. The CVD reactor of claim 4, wherein the bridgesegment is tubular.
 6. The CVD reactor of claim 1, wherein the highsurface area silicon filament includes an annular tube section having anouter diameter of at least 20 mm and a ratio of tube wall thickness toouter diameter of not greater than 1:4.
 7. The method of claim 1,wherein a wall thickness of the high surface area silicon filament isconstant along the length of the high surface area silicon filament towithin a tolerance of 10%.
 8. A CVD reaction system for bulk productionof polysilicon comprising: a CVD reactor having: a base plate systemconfigured with filament supports; an enclosure attachable to said baseplate system so as to form a deposition chamber; a power supply adjustedand configured for use with the solid rod silicon filament; electricalfeedthroughs in said base plate system, said electrical feedthroughsbeing adapted for connection of the power supply to said solid rodsilicon filament; a gas inlet in said base plate system connectable to asource of silicon-containing gas; and a gas outlet in said base platesystem whereby gas may be released from said chamber; a solid rodsilicon filament having a solid rod surface area; and a high surfacearea silicon filament having a surface area greater than the solid rodsurface area, said solid rod silicon filament and said high surface areafilament being usable interchangeably in the CVD reactor withoutreconfiguration or replacement of the power supply of the CVD reactor.9. The CVD reaction system of claim 8, wherein the cross sectional shapeof the high surface area silicon filament includes outwardly projectingfins.
 10. The CVD reaction system of claim 8, wherein the high surfacearea silicon filament is tubular.
 11. The CVD reaction system of claim8, wherein the high surface area silicon filament comprises two verticalfilament segments electrically connected by a bridge segment.
 12. TheCVD reaction system of claim 11, wherein the bridge segment is tubular.13. The CVD reaction system of claim 8, wherein the high surface areasilicon filament includes an annular tube section having an outerdiameter of at least 20 mm and a ratio of tube wall thickness to outerdiameter of not greater than ¼.
 14. The CVD reaction system of claim 8,wherein a wall thickness of the high surface area silicon filament isconstant along the length of the high surface area silicon filament towithin a tolerance of 10%.
 15. A method of modifying a CVD reactorsystem so as to increase its rate of bulk polysilicon production, theCVD reactor system including a reactor having: a base plate systemconfigured with filament supports; an enclosure attachable to said baseplate system so as to form a deposition chamber; a power supply adjustedand configured for use with a solid rod silicon filament having aspecified solid rod diameter, a specified solid rod length, and aspecified solid rod surface area; electrical feedthroughs in said baseplate system, said electrical feedthroughs being adapted for connectionof the power supply to both ends of said solid rod silicon filament; agas inlet in said base plate system connectable to a source ofsilicon-containing gas; and a gas outlet in said base plate systemwhereby gas may be released from said chamber, the method comprising:providing a high surface area silicon filament having a lengthsubstantially equal to the solid rod length and a surface area greaterthan the solid rod surface area, said high surface area filament havingdimensions that cause the high surface area filament to be usable in theCVD reactor without reconfiguring or replacing the power supply of theCVD reactor; installing the high surface area filament in the CVDreactor; introducing a silicon-containing gas into the depositionchamber through the gas inlet; and causing the power supply of the CVDreactor to apply electrical energy to the high surface area filamentwithout adjusting, reconfiguring, or replacing the power supply, therebyinitiating heating of the high surface area filament and causing siliconto be deposited from the silicon-containing gas onto the high surfacearea filament.
 16. The method of claim 15, wherein: the method furthercomprises, before installing the high surface area filament in the CVDreactor: installing a solid rod silicon filament in the CVD reactor,said solid rod silicon filament having a length substantially equal tosaid specified solid rod length, a diameter substantially equal to thespecified solid rod diameter, and a surface area substantially equal tothe solid rod surface area; attaching said enclosure to said base platesystem so as to form said deposition chamber; introducing asilicon-containing gas into the deposition chamber through the gasinlet; and causing the power supply of the CVD reactor to applyelectrical energy to the solid rod silicon filament without adjusting,reconfiguring, or replacing the power supply, thereby initiating heatingof the solid rod silicon filament and causing silicon to be depositedfrom the silicon-containing gas onto the solid rod silicon filament; andwherein installing the high surface area filament in the CVD reactorincludes removing the solid rod silicon filament from the CVD reactorand installing the high surface area filament in its place withoutreconfiguring or replacing the power supply of the CVD reactor.
 17. Themethod of claim 15, wherein the cross sectional shape of the highsurface area silicon filament includes outwardly projecting fins. 18.The method of claim 15, wherein the high surface area silicon filamentis tubular.
 19. The method of claim 15, wherein the high surface areasilicon filament comprises two vertical filament segments electricallyconnected by a bridge segment.
 20. The method of claim 19, wherein thebridge segment is tubular.
 21. The method of claim 15, wherein the highsurface area silicon filament includes an annular tube section having anouter diameter of at least 20 mm and a ratio of tube wall thickness toouter diameter of not greater than 1:4.
 22. The method of claim 15,wherein a wall thickness of the high surface area silicon filament isconstant along the length of the high surface area silicon filament towithin a tolerance of 10%.